1. Field of the Invention
The present invention relates to a center mode control circuit, and more specifically, to a center mode control circuit employed, for example, in a Dolby Pro Logic Surround Decoder, and suitable for an IC (Integrated Circuit).
2. Description of the Background Art
An article regarding to Dolby Pro Logic Surround System proposed by Dolby Laboratories Licensing Corp. of the United States appears on pp. 88-89 in "NIKKEI Electronics" issued on June 27, 1988. As shown in FIG. 2, the system is comprised of left and right input terminals 1 and 2 to which left and right stereo signals L.sub.T and R.sub.T are applied. A first adder circuit 3 adds the left and right stereo signals L.sub.T and R.sub.T to generate a sum signal C' (=L.sub.T +R.sub.T), a subtractor circuit 4 subtracts the right stereo signal R.sub.T from the left stereo signal LT to generate a difference signal S' (=L.sub.T -R.sub.T). First and second detection circuits 5 and 6 respectively detect levels of the left and right stereo signals L.sub.T and R.sub.T. Third and fourth detection circuits 7 and 8 respectively detect levels of the sum and difference signals C' and S'. A first level ratio detection circuit 9 level ratio of output signals of the first and second detection circuit 5 and 6. A second level ratio detection circuit 10 detects the level ratio of output signals of the third and fourth detection circuits 7 and 8. A VCA (Voltage Controlled Amplifier) 11 includes a plurality of gain controlled amplifier circuits (not shown), each controlling the level of the left or right stereo signal L.sub.T or R.sub.T in response to any one of the output signals of the first and second level ratio detection circuits 9 and 10. A second adder circuit 12 selectively adds the left and right stereo signals and output signals of the VCA 11 to generate left and right stereo output signals L and R, a center output signal C and a surround output signal S. A center mode control circuit 13 switches between the left and right stereo output signals L and R and the center output signal C in response to a mode, and a passive decoder 14 performs signal processing such as delay and noise reduction to the surround output signal S. Therefore, a structure such as shown in FIG. 2 enables a signal processing of enhancement of direction to clarify surround localization of sound, so that acoustics having presence can be provided to listeners Particularly, the system is effective when applied to an audio signal processing of a large-sized television, so that it can produce the same effect on an audience as is obtained in seeing a picture at a theater.
The center mode control circuit is disposed in order to switch for a normal mode, a phantom mode and a wide mode. In a normal mode, only a low frequency component of the center output signal C is added to the left and the right stereo output signals L and R. In a phantom mode, a full range of the center output signal C is added to the left and the right stereo output signals L and R. In a wide mode, nothing is added, and the left and the right stereo output signals L and R are generated as they are.
FIG. 3 is a circuit diagram showing an example of the conventional center mode control circuit, wherein left and right stereo input signals L and R, applied to left and right input terminals 15 and 16, are directly applied to first and second adder circuit 17 and 18, respectively. A center input signal applied to a center input terminal 19 is passed through a low pass filter 20 or a high pass filter 21, or directly transferred to terminals of first and second switches 22 and 23. The signal passed through the first switch 22 is applied to an attenuation circuit 24, wherein it is attenuated by 3 dB to be applied to the first and the second adder circuits 17 and 18, while the signal passed through the second switch 23 is provided to a center output terminal 25 as a center output signal C.sub.O.
States of the left and the left stereo output signals L.sub.O and R.sub.O and the center output signal C.sub.O in each mode will be described. In a normal mode, an output signal of the low pass filter 20 is applied to the first and the second adder circuits 17 and 18 through the first switch 22 and the attenuation circuit 24. As a result, the left stereo output signal L.sub.O becomes a signal obtained by addition of the left stereo input signal L and the high frequency cut off center input signal C.sub.L the right stereo output signal R.sub.O becomes a signal obtained by addition of the right stereo input signal R and a high frequency cut off center input signal C.sub.L and the center output signal C.sub.O becomes a high frequency center input signal C.sub.H. In a phantom mode, the center input signal C is applied to the first and the second adder circuits 17 and 18 through the attenuation circuit 24. As a result, the left stereo output signal L.sub.O becomes L+C, while the right stereo signal R.sub. O becomes R+C. In a wide mode, since the first switch 22 is opened while the second switch 23 selects the center input signal C, the left and the right stereo input signals L and R become the left and right stereo output signals L.sub.O and R.sub.O, respectively, and the center output signal C.sub.O becomes equal to the center input signal C.
Accordingly, the circuit of FIG. 3 enables an accurate center mode controlling.
However, the circuit of FIG. 3 had a problem in that it required the low pass filter 20 and the high pass filter 21, so that adjustment of characteristics of these filters was difficult. In addition, when this circuit is implemented as an integrated circuit, the low pass filter 20, the high pass filter 21 and the first and the second switches 22 and 23 should be externally attached to the IC, thereby increasing the number of pins for external attachment, so that it was not suitable for an integrated circuit.